How can I rotate the rigged child leg with whole body?
You could give your bones a root bone that would be the main parent of your bones (so it would be the parent of the current parent, that is probably the spone bone) and put the foot over this root when you'll want to make your rotation. Or create a bone under the foot and give your current main parent (first spine bone probably) a Child Of constraint that you activate (by keyframing the Influence value) only when you want the rotation to happen: